1. (Field of the Invention)
The present invention relates to a lens mount assembly for use in optical instruments such as photographic camera, compact cinecamera, video camera, copying machine and enlarger.
2. (Prior Art)
The zoom lens mount assembly, a specific type of the lens mount assembly, is well known.
The zoom lens mount assembly generally comprises a cam barrel including an elongate cam track formed through its cylindrical wall, and a movable barrel having a cam follower (specifically, a pin provided with a roller) slidably inserted into said track and telescopically received into said cam barrel, said movable barrel carrying therein a variable magnification lens and a compensating lens, wherein said cam barrel may be rotatably driven to move said movable barrel along the optical axis for adjustment of the optical magnification.
With such zoom lens mount assembly, the movable barrel is often provided at its front end with a focussing ring helicoidally threaded thereon so that said focussing ring carries a front lens assembly and may be rotated to adjust a position of said front lens assembly and thereby to achieve a desired focal adjustment.
In the zoom lens mount assembly which is recently available, said cam barrel is rotatably driven by an electric motor coupled by a gear to said cam barrel for automatic control of zooming and simultaneously said focussing ring is rotatably driven by another electric motor coupled by a gear to said focussing ring for automatic control of focussing.
In case of the well known zoom lens mount assembly as above mentioned, there is a problem particularly with respect to the gear coupling of the focussing ring projecting forward from the lens mount assembly, since the driving units respectively including the components such as the electric motors and the coupling gears are mounted on a stationary plate on which the lens mount assembly is mounted.
More specifically, the focussing ring is provided with a gear for focal adjustment and this gear is adapted to be engaged with the coupling gear of the associated driving unit. However, the position at which said focussing gear is engaged with the coupling gear of the driving unit is so spaced from said stationary plate that the support member for the focussing ring and the driving unit is subject to a deflection as a driving torque exerted upon the focussing ring increases, for example, said focussing ring is rotated to the point at infinity or the nearest position and, as a result, the engagement depth of the gear becomes shallow.
This is due to insufficient rigidity of the support member for the focussing ring and the driving unit, especially in the case when such support member is made of synthetic resin. This sometimes prevents a driving force from being accurately transmitted.
Upon occurrence of this effect, an image formation quality of the lens is disadvantageously deteriorated due to a deflection of the focussing ring.
Furthermore, the zoom lens mount assembly of prior art as has been described above is of an arrangement such that the movable barrel is moved along the optical axis for zooming and the focussing ring is rotated for focal adjustment. Such feature necessarily leads to increase of the movable parts and dust-proof means for these parts must be complicated.